For the next few weeks we will be running an experiment with HMTL5 video previews in the gallery. About half of the time you will get a dark HTML5 video player instead of the usual grey Flash-based player.
The hope is to make the shift to use only HTML5 video tags instead of the old Flowplayer solution. HTML5 video has the benefit that it works on most mobile devices too.

I love it when people support industry standards, I hope the experiment goes well.

I noticed a long delay before I was able to play the videos without any kind of indication why the play button wasn't actually working.

I suspect it was waiting for at least some of the video to be cached. After 10 seconds or so the video started playing.
I was able to start the video immediately the second time.
The video playback seemed fine once it had started.

I have seen the same behaviour 3 times with different sessions. I will post more info if I get a chance to experiment in future.

The buffering seems to be working better in chrome 34 today. There was a shorter dalay (4-5 seconds on sessions 2023 2024 and 2026).
During the delay the control bar was visible and changes to the control bar gave me some indication that the video was loading.

I am happy with the performance of the html5 <video>.

As for my preferences,
for similarity to flowplayer or other plugins:
1) It would be nice to have some kind of indicator showing that the video isn't ready to play for occasions where buffering takes an extended period of time. Since you don't have a lot of influence over the browser video player it could be an image or text external to the player that changes or is hidden when the player is ready (perhaps using "oncanplay" java event).

2) I would prefer to have the playback controls always visible when the video is in a window. If the controls are always visible the player height should be increased to accommodate the controls.

There should be a handful of FFMpeg distributions available for linux that will help with H264 decoding in common browsers.

I have used the FFDshow filters in Windows to get broad codec support in MS Media Player.

I believe Firefox and Opera prefer to support WebM and OGG formats as standard where Internet Explorer prefers MP4. Chrome reports to have support for all three.

I have seen examples where the HTML5 <video> block contains several <source> elements in different formats to cater for various browsers and even an <object> with a fall-back flash video for older versions of Internet Explorer.
With any luck the major browsers will all eventually support at least one common format. Until then it is up to each web master to decide on how many formats to serve and weather to require a plugin for browsers not supporting the preferred codec(s) natively :(.